I fitted a Roverdrive/Roamerdrive mechanism over Christmas 2015/16 and ran it for several years. It was certainly effective at lowering the revs at motorway speeds. You also don't get the sense of engine braking. when you lift off the accelerator that you do with standard gearing. Plus, as the revs are so low in overdrive and 5th the acceleration is a bit flat and you lose speed going up slopes.
The disadvantage I found was that it really heated up the transfer box oil. So much so that I added an oil cooler with a thermostatically controlled electric pump after six months. I tapped a hole in the sump to screw in a temperature sender unit and this activated a relay to turn the pump on. It worked quite well to keep the temperature in the recommended range on long motorway trips. Despite this the Roamerdrive started overheating the transfer box again towards the end of last year. I'm not running it at the moment. It's currently in the shed awaiting a moment when curiosity and spare time afford the opportunity to dismantle it. I read that bearing failures are not unknown, so maybe that's what has happened. The makers claim that all the bearing races are standard sizes, so can be sourced from anybody that sells engineering supplies.
In the meantime, in January/February this year I fitted a reconditioned R380 gearbox as the old one was getting extremely noisy. With the new one, I requested a high 5th gear ratio. This is if anything better than the overdrive unit as it gives you a lot more flexibility at around 65-75 mph, so it's a lot easier to join fast moving traffic from a slip road, change into a stream of traffic in lane 2 or 3 and so on, because you can just mirror, signal, shoulder check and accelerate into a gap, just like a Youtube advanced driving video, rather than having to wait ages to pick up speed as with the overdrive. Even so, with my new R380, a modest 3000 rpm would take me well past the ACPO charging threshold for any UK speed limit.
I got 60,000 miles out of the Roamerdrive and it probably just needs a bearing or two. Most of that 60,000 miles would have been done in overdrive, as most of my miles are on motorways and A class roads.
I have found that the major influences on fuel economy is not so much the gearing but other factors like headwinds, whether I've got stuff on the roof rack, and traffic jams. The best way to optimise fuel economy with the TD5 is to keep it moving. I think that while the engine's going more slowly in overdrive, the ECU is running it a bit richer because it's not getting the rpm at the speed sensor that it would expect, the same as if you were towing a trailer or going uphill. Whereas in a lower gear the engine might be turning faster but running leaner.
